U.S. patent application number 13/983996 was filed with the patent office on 2014-01-23 for current limitation system for limiting the effects of a fault in a dc grid and a method of operating a current limitation system.
This patent application is currently assigned to ABB TECHNOLOGY AG. The applicant listed for this patent is Bertil Berggren, Jurgen Hafner, Lars-Erik Juhlin, Kerstin Linden, Lidong Zhang. Invention is credited to Bertil Berggren, Jurgen Hafner, Lars-Erik Juhlin, Kerstin Linden, Lidong Zhang.
Application Number | 20140022680 13/983996 |
Document ID | / |
Family ID | 44625374 |
Filed Date | 2014-01-23 |
United States Patent
Application |
20140022680 |
Kind Code |
A1 |
Berggren; Bertil ; et
al. |
January 23, 2014 |
CURRENT LIMITATION SYSTEM FOR LIMITING THE EFFECTS OF A FAULT IN A
DC GRID AND A METHOD OF OPERATING A CURRENT LIMITATION SYSTEM
Abstract
A current-limitation system for limiting a current through an DC
connection in case of a fault occurring in a DC grid of which the
DC connection forms a part is provided, as well as a method of
operating a current-limitation system for limiting a current
through an DC connection in case of a fault occurring in a DC grid
of which the DC connection forms a part.
Inventors: |
Berggren; Bertil; (Vasteras,
SE) ; Hafner; Jurgen; (Ludvika, SE) ; Juhlin;
Lars-Erik; (Ludvika, SE) ; Linden; Kerstin;
(Ludvika, SE) ; Zhang; Lidong; (Vasteras,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Berggren; Bertil
Hafner; Jurgen
Juhlin; Lars-Erik
Linden; Kerstin
Zhang; Lidong |
Vasteras
Ludvika
Ludvika
Ludvika
Vasteras |
|
SE
SE
SE
SE
SE |
|
|
Assignee: |
ABB TECHNOLOGY AG
ZURICH
CH
|
Family ID: |
44625374 |
Appl. No.: |
13/983996 |
Filed: |
March 11, 2011 |
PCT Filed: |
March 11, 2011 |
PCT NO: |
PCT/EP2011/053738 |
371 Date: |
September 10, 2013 |
Current U.S.
Class: |
361/79 ;
361/87 |
Current CPC
Class: |
H02H 7/268 20130101;
H02H 3/08 20130101; H02H 3/20 20130101; H02H 3/02 20130101 |
Class at
Publication: |
361/79 ;
361/87 |
International
Class: |
H02H 3/08 20060101
H02H003/08; H02H 3/20 20060101 H02H003/20 |
Claims
1-23. (canceled)
24. A current-limitation system for limiting a current through a DC
connection in case of a fault occurring in a DC grid of which the
DC connection forms a part, the current-limitation system
comprising a current limiter for series-connection into the DC
connection, and a control system for controlling the
current-limiting strength of the current limiter; wherein the
current limiter comprises a series connection of independently
controllable breaker sections, wherein a breaker section comprises
a parallel connection of a non-linear resistor and a semi-conductor
switch of turn-off type; the control system comprises a current
measuring device; and the control system is operable to: detect a
fault; and, in response to detecting the fault, adjust the
current-limiting strength of the current limiter, the
current-limitation system being wherein the control system further
comprises a voltage measuring device arranged to measure the
voltage at at least one side of the current limiter; the control
system is operable to detect a fault by checking whether the
voltage at at least one side of the current limiter has fallen
below a first voltage threshold; the current measuring device is
arranged to measure the current through the current limiter; and
the control system is operable to adjust, in response to detection
of a fault, the current-limiting strength of a current limiter in a
manner so that if the current through the current limiter exceeds a
first current threshold (I.sub.max), the current-limiting strength
is increased; and if the current through the current limiter falls
below a second current threshold level (I.sub.min), the
current-limiting strength is decreased, wherein the first current
threshold lies below the rated current of the transmission.
25. The current-limitation system of claim 24, wherein the control
system is further operable to: estimate the present energy
absorbing capacity of the non-linear resistors; and select which
breaker section(s) should be opened or closed, if any, in
dependence of the different present energy absorbing capacities of
the non-linear resistors.
26. The current-limitation system of claim 25, wherein the current
limiter further comprises a transfer switch connected in parallel
with the series-connection of breaker sections, and the control
system is further operable to open the transfer switch, thus
commutating the current to the series connection of breaker
sections, if: the current through the current limiter exceeds a
third current threshold (I.sub.arm), wherein the third current
threshold is above the expected current level during normal
operation, or the voltage at an end of the current limiter falls
below a second voltage threshold, wherein the second voltage
threshold is below the expected voltage level during normal
operation.
27. The current-limitation system of claim 24, wherein the current
limiter is a current limiting breaker capable of breaking the
current at at least the rated voltage.
28. The current-limitation system of claim 24, further comprising a
self-protective control system operable to generate a damage
indication if the current limiter is at risk for thermal damage,
and to generate, in response to such damage indication, a tripping
signal instructing the current limiter, or a breaker protecting the
current limiter, to break the current.
29. An AC/DC converter station comprising an AC/DC converter and a
connection to a DC grid, wherein the AC/DC converter station
further comprises a current-limitation system according to claim
24, wherein the current limiter of the current-limitation system is
series-connected in the connection to the DC grid.
30. A DC grid comprising at least two AC/DC converters
interconnected via a DC connection, the DC grid further comprising:
at least one current-limitation system according to claim 24, where
the current limiter of the current-limitation system is
series-connected in a DC connection.
31. The DC grid of claim 30, wherein the DC grid comprises a
plurality of AC/DC converters interconnected via DC lines, and
wherein the DC grid is divided into at least two zones by means of
at least one current limiting system in a manner so that a current
limiter is connected in each of the DC line(s) by which two zones
are interconnected.
32. A method of operating a current-limitation system for limiting
a current through a DC connection in case of a fault occurring in a
DC grid of which the DC connection forms a part, the
current-limitation system comprising a current limiter and a
control system, wherein the current limiter comprises a number of
individually controllable, series connected breaker sections, each
comprising a parallel connection of a non-linear resistor and a
semi-conductor switch of turn-off type; the method comprising:
detecting, in the current-limitation system, a fault; and, in
response to detecting the fault, adjusting the current-limiting
strength of the current limiter; the method being wherein: the
detecting of the fault is performed by checking whether the voltage
at at least one side of the current limiter has fallen below a
first voltage threshold; and the adjusting of the current-limiting
strength of the current limiter is performed in a manner so that if
the current through the current limiter exceeds a first current
threshold (I.sub.max), the current-limiting strength is increased;
and if the current through the current limiter falls below a second
current threshold level (I.sub.min), the current-limiting strength
is decreased, wherein the first current threshold lies below the
rated current of the transmission.
33. The method of claim 32, further comprising estimating the
present energy-absorbing capacity of the non-linear resistors; and
selecting which breaker section(s) should be open or closed, if
any, in dependence of the different energy absorbing capacities of
the non-linear resistors.
34. The method of claim 32, wherein the current limiter comprises a
transfer switch connected in parallel with a main switch; said step
of adjusting the current-limiting strength is conditional on the
transfer switch being open; and the method further comprises:
opening the transfer switch, thus commutating the current to the
main switch, if: the current through the current limiter exceeds a
third current threshold (L.sub.arm), wherein the third current
threshold is above the expected current level during normal
operation, or the voltage at a side of the current limiter goes
below a second voltage threshold, wherein the second voltage
threshold is below the expected voltage level during normal
operation.
35. The method of claim 32, further comprising generating a damage
indication in dependence of estimations of the present
energy-absorbing capacity of the current limiter indicating that
the current limiter is at thermal risk; and tripping the current
limiter to break the current in response to said damage
indication.
36. A computer program for operating a limitation-determination
system for controlling the current-limiting strength of a current
limiter which is series-connected in a DC connection in order to
limit the effects of a fault in a DC grid of which the DC
connection forms a part, the current limiter comprising a series
connection of independently controllable breaker sections, wherein
a breaker section comprises a parallel connection of a non-linear
resistor and a semi-conductor switch of turn-off type, the computer
program comprising: computer program code portions which, when run
on a processor of the a limitation-determination system, causes the
limitation-determination system to: receive measurements of a
current through the current limiter; receive measurements of the
voltage at at least one side of the current limiter; check whether
a fault in the DC grid can be detected using said voltage
measurements; and, in response to detection of a fault: adjust the
current-limiting strength of the current limiter in a manner so
that if the current through the current limiter exceeds a first
current threshold (I.sub.max), the current-limiting strength is
increased; and if the current through the current limiter falls
below a second current threshold (I.sub.min), the current-limiting
strength is decreased, wherein the first current threshold lies
below the rated current of the transmission.
37. A DC grid comprising a plurality of AC/DC converters
interconnected via DC lines, wherein: the DC grid is divided into
at least two zones by means of at least one current-limitation
system comprising a current limiter and a control system for
controlling the current-limiting strength of the current limiter,
the DC grid being divided in a manner so that a current limiter is
series connected in each of the DC line(s) by which two zones are
interconnected; a current limiter of a current-limitation system
comprises a series connection of independently controllable breaker
sections, wherein a breaker section comprises a parallel connection
of a non-linear resistor and a semi-conductor switch of turn-off
type; the control system of a current-limitation system comprises a
measuring device arranged to measure the current through the
current limiter; said control system is operable to: detect a
fault; and, in response to detecting the fault: adjust the
current-limiting strength of the current limiter in a manner so
that if the current through the breaker exceeds a first current
threshold (I.sub.max), the current-limiting strength is increased;
and if the current through the breaker falls below a second current
threshold level (I.sub.min), the current-limiting strength is
decreased.
38. The current-limitation system of claim 25, wherein the current
limiter is a current limiting breaker capable of breaking the
current at at least the rated voltage.
39. The current-limitation system of claim 26, wherein the current
limiter is a current limiting breaker capable of breaking the
current at at least the rated voltage.
40. The current-limitation system of claim 25, further comprising a
self-protective control system operable to generate a damage
indication if the current limiter is at risk for thermal damage,
and to generate, in response to such damage indication, a tripping
signal instructing the current limiter, or a breaker protecting the
current limiter, to break the current.
41. The current-limitation system of claim 26, further comprising a
self-protective control system operable to generate a damage
indication if the current limiter is at risk for thermal damage,
and to generate, in response to such damage indication, a tripping
signal instructing the current limiter, or a breaker protecting the
current limiter, to break the current.
42. The current-limitation system of claim 27, further comprising a
self-protective control system operable to generate a damage
indication if the current limiter is at risk for thermal damage,
and to generate, in response to such damage indication, a tripping
signal instructing the current limiter, or a breaker protecting the
current limiter, to break the current.
43. An AC/DC converter station comprising an AC/DC converter and a
connection to a DC grid, wherein the AC/DC converter station
further comprises a current-limitation system according to claim
25, wherein the current limiter of the current-limitation system is
series-connected in the connection to the DC grid.
Description
TECHNICAL FIELD
[0001] The present invention relates to the field of power
transmission, and in particular to power transmission using High
Voltage Direct Current (HVDC) technology.
BACKGROUND
[0002] Transmission of power over long distances can advantageously
be performed using HVDC transmission lines. In an AC transmission
system, the transmission losses are dependent on both active and
reactive power transfers. For long transmission lines, the losses
due to the reactive power transfer will be significant. In an HVDC
transmission system, on the other hand, only active power is
transferred. The losses in an HVDC transmission line will thus be
lower than the losses in an AC transmission line of the same
length. For long distance transmission, the higher investment of
necessary conversion equipment in an HVDC system is often
justified.
[0003] A drawback of DC transmission as compared to AC transmission
is that the interruption of a fault current is more difficult. A
fault current in an AC system inherently exhibits frequent zero
crossings, which facilitate for fast current interruption. In a DC
system, no inherent zero crossings occur. In order to break a DC
current, a zero crossing of the DC current generally has to be
forced upon the system.
[0004] Moreover, in an AC system, the fault current will be limited
by the reactance of the transmission lines. In a DC system on the
other hand, the inductance of a transmission line will only matter
in the transient stage. When the transient (quite quickly)
diminishes, only the resistance of the lines will limit the level
of the fault current on the DC side. Thus, the fault current can
grow very rapidly in a DC grid. A fast breaking of a fault current
is therefore desired.
[0005] Furthermore, power from the AC side will be fed into a fault
that occurs on the DC side. Typically, this implies that the fault
currents are high on the DC side, whereas the DC voltages in case
of a fault will be low throughout the DC grid, making organized
power transfer impossible during the faulted time period. This is
particularly pronounced when at least some of the converters are
based on Voltage Source Converter (VSC) technology, since the
switches of a VSC converter will typically have to be blocked when
the current rises above a certain level, leaving the VSC converter
basically operating as a diode bridge. This level is here referred
to as the converter blocking-level. The more converters that are
connected to the DC grid, the higher the DC current in the fault.
The situation of having depressed DC voltages, with the
consequential power transfer inability, may, if prolonged, have
serious impact on the AC system stability. AC system instability
would result in black-outs, which are very costly for society. In
order to prevent AC system instability, the AC systems could be
designed with substantial reserve transfer capability. However,
such over-dimensioning of the AC systems is very costly and
generally not desired. Hence, a fast breaking of a DC fault
current, before the DC voltages have collapsed, is desired.
[0006] Thus, in order to limit the effects of a line fault, an HVDC
breaker should react very fast, typically in the transient stage
while the fault current still is increasing and before the DC
voltages have collapsed too much. Efforts have been put into the
development of fast and reliable HVDC breakers, and the HVDC
breakers that currently provide the fastest interruption of current
are based on semi-conducting technology. A semi-conductor HVDC
breaker is for example disclosed in EP0867998. However,
semi-conductor HVDC breakers experience a power loss which is
higher than in a mechanical breaker. Furthermore, semi-conductor
HVDC breakers designed to break large currents are considerably
more expensive than mechanical breakers. However, existing
mechanical breakers cannot provide sufficient breaking speed. Thus,
there is a need for cost- and energy effective fault current
handling in a DC grid.
SUMMARY
[0007] A problem to which the present invention relates is how to
efficiently limit the negative consequences of a fault occurring in
a DC grid.
[0008] This problem is addressed by a current-limitation system for
limiting a current through a DC connection in case of a fault
occurring in a DC grid of which the DC connection forms a part. The
current-limitation system comprises a current limiter for
series-connection into the DC connection, and a control system for
controlling the current-limiting strength of the current limiter.
The control system comprises the control system comprises a current
measuring device arranged to measure the current through the
current limiter, and the control system is operable to detect a
fault. The control system is further operable to, in response to
detecting the fault; to adjust the current-limiting strength of the
current limiter in a manner so that if the current through the
breaker exceeds a first current threshold (I.sub.max), the
current-limiting strength is increased; and if the current through
the breaker falls below a second current threshold (I.sub.min), the
current-limiting strength is decreased.
[0009] The problem is further addressed by a method of operating a
current-limitation system for limiting a current through a DC
connection in case of a fault occurring in a DC grid of which the
DC connection forms a part. The method comprises detecting, in the
current-limitation system, a fault. The method further comprises,
in response to detecting the fault, adjusting the current-limiting
strength of the current limiter in a manner so that if the current
through the breaker exceeds a first current threshold (I.sub.max),
the current-limiting strength is increased; and if the current
through the breaker falls below a second current threshold
(I.sub.min), the current-limiting strength is decreased.
[0010] In one embodiment, the first current threshold lies below
the rated current of the transmission. This embodiment can for
example be advantageous when the current limiter is connected at
the DC side of an AC/DC converter to limit the current flowing from
the AC/DC converter into a fault in the DC grid. By limiting the
fault current from the AC/DC converter to a level below the rated
current, the active power transfer to the DC grid from the AC/DC
converter will be kept below the rated power of the AC/DC
converter, thus allowing for the provision of reactive power from
the AC/DC converter to an AC power system connected at the AC side
of the AC/DC converter.
[0011] In another embodiment, the second current threshold lies
above the rated current of the transmission (while the first
current threshold lies below the fault current that occur had no
current limiter been present). This embodiment can for example be
advantageous when a quick recharging of the transmission lines is
desired upon clearing of the fault. This can for example be the
case if the current limitation system is used to divide a DC grid
into at least two zones, in order to prevent the effects of a fault
in a zone to spread outside that zone.
[0012] Regulation of a fault current to lie within a regulation
range falling below or above the rated current can be achieved in
dependence on voltage measurement at the side of the current
limiter which faces the fault, and in dependence on measurements of
the current through the current limiter.
[0013] Regulation of a fault current to lie within a regulation
range falling above the rated current can be made in dependence on
the current through the current limiter. The method of operation of
the current limiter could then be simplified in that no voltage
measurements are required.
[0014] In one embodiment the current limiter comprises a series
connection of independently controllable breaker sections, wherein
a breaker section comprises a parallel connection of a non-linear
resistor and a semi-conductor switch of turn-off type. The control
system is in this embodiment operable to send blocking and/or
firing signals to the semiconductor switches of the breaker
sections in order to adjust the current-limiting strength of the
current limiter. By this embodiment is achieved that an efficient
adjustment of the current-limiting strength of the current limiter
is achieved. In one implementation of this embodiment, the control
system is further operable to estimate the present energy absorbing
capacity of the non-linear resistors; and select which breaker
section(s) should be opened or closed, if any, in dependence of the
different present energy absorbing capacities of the non-linear
resistors. An efficient utilization of the energy-absorbing
capacity of the current limiter is thus achieved.
[0015] In this embodiment, the current limiter could comprise a
transfer switch which is closed during normal operation, but which
is opened to commutate the current to the series-connection of
breaker sections in case of a fault. By use of a transfer switch
wherein the power loss is lower than in the series-connection of
breaker sections, the power loss in the current limiter can be
reduced during normal operation.
[0016] The current-limiter could be a current-limiting breaker
capable of breaking the current at at least the rated voltage. The
current-limitation system could further comprise a self-protective
control system operable to generate a damage indication if the
current limiter is at risk for thermal damage, and to generate, in
response to such damage indication, a tripping signal instructing
the current limiter, or a breaker protecting the current limiter,
to break the current. Such damage indication could for example be
based on estimates of the present energy-absorbing capability of
the non-linear resistors, when the current limiter comprises a
series connection of breaker sections.
[0017] The problem is further addressed by an AC/DC converter
station comprising a AC/DC converter, a connection to a DC grid and
a current-limitation system, wherein the current limiter of the
current-limitation system is series-connected in the connection to
the DC grid. The problem is also addressed by a DC grid (100)
comprising at least two AC/DC converters interconnected via a DC
connection, as well as at least one current-limitation system
according to any one of the above claims, where the current limiter
of the current-limitation system is series-connected in a DC
connection. In one embodiment of the DC grid, the DC grid is
divided into at least two zones by means of at least one current
limiting system in a manner so that a current limiter is connected
in each of the DC line(s) by which two zones are
interconnected.
[0018] Further aspects of the invention are set out in the
following detailed description and in the accompanying claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a schematic drawing of an example of a DC
grid.
[0020] FIG. 2 is a schematic drawing of the DC grid of FIG. 1,
where the current limiters have been series connected in the
connections between the HVDC converters and the DC switchyards.
[0021] FIG. 3 schematically illustrates a DC grid wherein current
limiters are used to divide the grid into two zones, and wherein
the HVDC converters of one zone are connected to the DC grid via
current limiters.
[0022] FIG. 4a illustrates an example of a current limiter based on
a series connection of independently controllable breaker
sections.
[0023] FIG. 4b illustrates an example of a current limiter
including a transfer switch.
[0024] FIG. 5 illustrates an example of current-limitation system
comprising a current-limiter and a control system arranged to
control the current-limiting strength of the current limiter, where
the control system comprises a limitation-determination system.
[0025] FIG. 6a is a flowchart illustrating an embodiment of a
method of determining the required current-limiting strength of a
current limiter for the case when the regulation range lies above
the rated current of the transmission.
[0026] FIG. 6b is a flowchart illustrating an embodiment of a
method of determining the required current-limiting strength of a
current limiter for the case when the maximum current of the
regulation range lies above or below the rated current of the
transmission.
[0027] FIG. 7 is an example of a configuration wherein a current
limiter is connected in series with a reactor to reduce the time
derivative of the current upon regulation of the current-limiting
strength.
[0028] FIG. 8a is a flowchart illustrating an example of a method
of controlling a transfer switch.
[0029] FIG. 8b shows a flowchart illustrating another example of a
method of controlling a transfer switch.
[0030] FIG. 9 shows a flowchart illustrating an embodiment of a
process for generating control signals to send to a current limiter
once the required limiting strength has been determined.
[0031] FIG. 10 schematically shows an alternative illustration of a
limitation-determination system shown in FIG. 5.
[0032] FIGS. 11a-d illustrate the sequence of events in terms of
current and voltage as a function of time for different embodiments
of the method of determining the required current limiting strength
of a current limiter.
DETAILED DESCRIPTION
[0033] FIG. 1 is a schematic illustration of an example of a DC
grid 100 for high voltage DC (HVDC) transmission. The DC grid 100
comprises five different high voltage AC/DC converters 105, here
referred to as HVDC converters 105, which are interconnected via DC
switchyards 120 and DC lines 115 for high voltage transmission,
here referred to as HVDC lines 115. A HVDC converter 105 is
connected to an AC power system (not shown) at one end, and to a DC
switchyard 120 via a connection 110 at the other end. In FIG. 1,
the DC switchyards 120 have been shown to be of a single bus bar
configuration for illustration purposes, but other configurations
may alternatively be used, such as a double busbar, a two breaker
switchyard, a one and a half breaker switchyard, etc. The HVDC
lines 115 may be cable or overhead lines, or combinations thereof.
Connections 110 and HVDC lines 115 can be bipolar or
mono-polar.
[0034] A converter 105 could for example be a Voltage Source
Converter (VSC), or a Current Source Converter (CSC). In recent
years, point-to-point HVDC transmission systems based Voltage
Source Converter (VSC) have been developed. VSC technology is
particularly advantageous for building DC grids, inter alia since
the VSC technology allow for power reversal by simply performing a
DC current reversal.
[0035] A connection 110 in a high voltage DC grid is typically
connected to a DC switchyard 120 over a high voltage DC breaker
130i, hereinafter referred to as an HVDC breaker 130i. A connection
between the HVDC converter 105 and an AC switchyard (at the other
side of the HVDC converter 105) is typically made over an AC
breaker (not shown). An HVDC breaker 130ii is typically provided at
the connection of a HVDC line 115 to a DC switchyard 120, so that
each HVDC line 115 is equipped with two HVDC breakers 130ii, which
are located at the respective ends of the HVDC line 115. In the
following, when referring to HVDC breakers generally, the term HVDC
breaker 130 will be used. The various HVDC breakers 130i and 130ii
could, if desired, be implemented in the same manner, and the
difference in reference numeral only indicates the difference in
location in the DC grid topology.
[0036] A protection system 135 is typically provided at each
switchyard 120, the protection system 135 being designed to detect
a fault situation and to send, if required, a trip signal to the
appropriate HVDC breaker(s) 130. Thus, in case of a fault on a HVDC
line 115, the HVDC breakers 130ii at each end of the HVDC line 115
will receive a trip signal from such protection system 135. A line
fault could e.g. be a pole-to-ground fault or a pole-to-pole fault,
or a combination thereof. Similarly, if a fault is detected on the
connection 110, or in the HVDC converter 105, the HVDC breaker 130i
will receive a trip signal. In these latter scenarios, an AC
breaker on the AC side will also receive a trip signal. However, in
the following, for ease of description, reference will be made only
to the tripping of HVDC breakers 130. A protection system 135 is
typically designed to only remove the faulty piece of equipment in
case of a fault, leaving the rest of the system intact after fault
clearing. Protection systems are well known in the art, and
typically comprise measurement equipment and software algorithms
for the determination of existence of a fault. Such algorithms
could for example be based on measurements of voltage and current,
their magnitudes and/or their derivatives in various combinations,
and/or on so called differential protection, which is based on
comparisons of the current at each side of an object, such as an
HVDC line 115 or a HVDC converter 105.
[0037] The DC grid 100 of FIG. 1 is an example only, and a DC grid
100 could here comprise any number N of HVDC converters 105, where
N.gtoreq.2, interconnected in any fashion. A DC grid 100 typically
comprises further equipment which has not been shown in FIG. 1,
such as measurement devices, DC reactors, filters, etc. When only
two HVDC converters 105 are interconnected in a point-to-point
transmission line, no DC switchyard 120 will have to be
provided.
[0038] The clearing of a fault will be performed by tripping of the
appropriate HVDC breakers 130 surrounding the faulty object. Such
tripping will typically be initiated by the protection system 135
which monitors the faulty object. Objects wherein a fault could
occur could for example be an HVDC line 115, an HVDC converter 105,
a connection 110 or a DC switchyard 120.
[0039] As mentioned above, the speed at which the HVDC breakers 130
operate will determine how high a fault current will rise in the DC
grid 100 before it is broken. It is generally desired to keep the
breaking speed as high as possible. Today, HVDC breakers based on
semi-conductor technology can be made sufficiently fast, with
breaking speeds of as low as in the .mu.s scale. However, since
semi-conducting HVDC breakers based on power electronic technology
are typically quite costly compared to less speedy alternatives, a
way of efficiently limiting the negative consequences of a fault
occurring in a DC grid 100, while using more slowly operating HVDC
breakers, would be desired.
[0040] According to the invention, a current limiter for limiting a
current through a connection 110 or a HVDC line 115 in a DC grid is
provided. A control system for controlling the current-limiting
strength of the current-limiter is also provided, the current
limiter and the control system forming a current-limitation system.
The current limiter has an interface for series-connection into a
connection 110 or an HVDC line 115. In the following, when commonly
referring to connections 110 and HVDC lines 115, the term HVDC
connection will be used, the term DC connection including both DC
lines 115 and connections 110 connecting an AC/DC converter 105 to
a DC grid 100.
[0041] By use of a fast current limiter, other HVDC breakers 130
within the DC grid 100 can be of a design which provides slower
operation. By quickly limiting a fault current to a level lower
than what the fault current would have been if no current limiter
was present, HVDC breakers 130 of comparatively low breaking speed
can be used. Furthermore, the breaking capacity of at least some
HVDC breakers 130 in the DC grid 100 can be reduced, since the
currents to be broken, even if a fault situation occurs, will be
lower. Depending on at which position(s) in the DC 100 grid that
the fast current limiter(s) are provided, and at what positions
HVDC breakers 130 are provided, the positive effects of the current
limitation provided by the current limiter(s) will benefit
different HVDC breakers 130 at different positions, in the DC
grid.
[0042] The breaking speed requirements on a HVDC breaker 130 in a
DC grid 100 depends for example on for how long the current limiter
205 can operate to hold the current at an acceptable level; on the
stability of an AC system; to which current level the current
limiters 205 control the current; and on the current-breaking
capability on the HVDC breaker 130. Examples of suitable designs of
HVDC breakers of lower breaking speed are e.g. mechanical HVDC
breaker designs such as those described in "Cigre technical
brochure 114, Circuit-breakers for meshed multiterminal HVDC
systems", the breaking speed of which is in the range of an
AC-breaker, e.g. 30-60 ms.
[0043] Fast current limiters could also be used to facilitate for a
DC grid 100 having few or no HVDC breakers 130. By liming the fault
current in case of a fault, fault clearing could at least partly be
performed by AC breakers connected at the AC side of the HVDC
converters 105. In this implementation of a DC grid 100, the
effects of a fault will generally be spread over a larger
geographical area of the DC grid 100, than if each object in the DC
grid is protected by means of HVDC breakers 130. However, the cost
related to providing HVDC breakers 130 for protecting the different
objects (such as HVDC lines and HVDC converters) can be reduced or
avoided. Examples of objects which are often protected by means of
HVDC breakers 130 are HVDC lines 115, connections 110, and HVDC
converters 105.
[0044] A current limiter could advantageously be series-connected
in the connection 110 at the DC side of a HVDC converter 105. When
DC switchyards 120 are provided, this location of the current
limiter would be in the connection 110 between the HVDC converter
105 and the nearest DC switchyard 120. In this embodiment of a DC
grid 100, the current limiter can efficiently limit the
contribution from the HVDC converter 105, to which it is connected,
to a fault current into the DC grid 100. Thus, upon occurrence of a
fault, in the time period between activation of the current limiter
and breaking of the current by HVDC breakers surrounding the fault,
the current flowing into the fault from the HVDC converter 105 will
be limited. Furthermore, the voltage at the DC side of the
converter 105 will be held up by means of the current limiter, thus
reducing the effects of the fault on the AC system connected at the
AC side of the HVDC converter 105.
[0045] An example of a DC grid wherein current limiters 205 are
series-connected at the DC side of the HVDC converters 105 is
schematically illustrated in FIG. 2. In FIG. 2, a current limiter
205 has been connected at the DC side of all HVDC converters 105 of
the DC grid 100. A HVDC converter 105, the connection 110 and the
current limiter 205 connected at the DC side of the HVDC converter
105 could be seen to be part of an HVDC converter station. In
another implementation of a DC grid, less than all HVDC converters
105, for example only one HVDC converter 105, could have a current
limiter 205 connected on its DC side. In FIG. 2, no HVDC breaker
130i is shown in the connections 110 which are provided with a
current limiter 205. However, an HVDC breaker 130i could
additionally be provided in series with the current limiter 205.
Such HVDC breaker 130i could for example be beneficial in order to
limit the effects on the DC grid 100 from a fault in the connection
110 or in the HVDC converter 105 if a uni-directional current
limiter 205 is used. Since, in case of a fault anywhere in the DC
grid 100, the current will flow in the direction from the HVDC
converters 105 towards the DC grid (unless the fault occurs between
the HVDC converter 105 and the current limiter 205), this is the
current direction in which current limiting will be most
beneficial. Hence, a current limiter 205 in a connection 110 could
be uni-directional with maintained performance, thus saving
components as compared to a bi-directional current limiter 205.
However, a bi-directional current limiter 205 could also be used.
Furthermore, for illustration purposes, no protection systems 135
have been shown in FIG. 2, although such protections systems would
typically be present.
[0046] A line fault 200 on an HVDC line 115 has been shown in FIG.
2. A line fault is only an example of different faults, the effects
of which a current limiter can mitigate.
[0047] As mentioned above, by use of a current limiter 205 in the
connection 110, the current flowing into the DC grid from the HVDC
converter 105 can be kept at a lower level, thus reducing the
stress on the components of the DC grid 100, for example on the
HVDC breakers 130 which will be used to clear the fault. Moreover,
as will be further discussed in relation to FIGS. 6a and 6b, the
effects of a current limiter 205 in the connection 110 can be of
great benefit also for the operation of the HVDC converter 105 and
the AC power system connected to the AC side of the HVDC converter
105.
[0048] Current limiters can further be useful also at other
locations in the DC grid 100. FIG. 3 illustrates an example of a DC
grid 100 wherein the grid has been divided into two zones 300 (zone
300:1 and zone 300:2) by series-connecting a current limiter 205 in
each HVDC line(s) 115z by which two zones 300 are interconnected.
In FIG. 3, a zone-dividing current limiters 205 are shown to have
taken the place of an HVDC breaker 130i. However, an additional
HVDC breaker 130i could be provided in series with the
zone-dividing current limiter 205. This could for example be
beneficial if the current limiter 205 does not have capacity to
break a fault current. A zone-dividing current limiter 205 could
advantageously be bi-directional, in order to allow for limitation
of a fault current flowing into either of the zones 300
interconnected by the current limiter 205.
[0049] By dividing the DC grid into different zones which are
interconnected by means of a current limiter, the effects of a
fault in the DC grid can be limited. If a fault occurs in a first
zone, the current limiter(s) that interconnect this first zone 300
with its neighbouring zones will limit the fault current that flows
into the fault from the neighbouring zones during the fault-on
period, thus reducing the fault current in the zone wherein the
fault has occurred. By providing a limited contribution to the
fault current through a zone-dividing HVDC line 115z, current from
the zones 300 which surround the faulty zone 300 will contribute to
the charging of the cables and/or overhead lines in the faulty
zone, following clearing of the fault. Thus, normal operation can
quickly be resumed also in the zone where the fault occurred, once
the faulty object has been disconnected. Moreover, limiting of the
fault current will mitigate the effects in the healthy zones
surrounding the faulty zone. If the fault current is permitted to
flow undisturbed, the DC voltage in the surrounding zones 300 will
collapse, making continued power transmission more or less
impossible. Such voltage collapse will typically, unless measures
are taken, reach a large geographical spread very quickly after a
fault has occurred. By limiting the fault-current caused by a fault
in a first zone, the DC voltage in the surrounding zones 300 can be
essentially undisturbed, and the power transmission in the
surrounding zones 300 may continue without major interruption.
[0050] Hence, the impact of the fault on AC system stability can be
well kept under control. In other words, the power transfer can be
maintained in a large portion of the DC grid 100, even if a fault
in the DC grid has occurred. Thus, the reserve transfer capacity in
the connected AC systems can be substantially decreased compared to
a system wherein no current limiters 205 have been implemented in
the DC grid. In addition, the fault-on period with depressed
voltages and inability to transfer power in the faulty zone can be
allowed to be longer and thus, slower and less expensive HVDC
breakers 130 can be used within the zones 300.
[0051] By combining a current-limiting possibility in the
connections 110 with the division of the DC grid 100 into different
zones 300, the fault current in a zone 300, wherein a fault has
occurred, can be efficiently controlled to an acceptable level.
[0052] Once the fault current has been limited by means of the
current limiters 205, the HVDC breakers 130 which are connected
around the faulty object can clear the fault. The instruction to
commence closing the breaker sections 400 will be generated locally
by the control system of the respective current limiters 205, as
further described below.
[0053] The control system arranged to activate current limitation
of a current limiter 205 is advantageously independent on the
protection systems 135, and vice versa. The tripping of the HVDC
breakers 130 in response to a fault will thus be independent on the
activation of the current limiter 205. Depending on the
implementation, the tripping of the HVDC breakers 130 will be
initiated at the same time as, before, or after the activation of
the current limiters 205. The activation and control of the current
limiter 205 can advantageously be trigger in dependence of local
measurements of voltage/and or current, obtained at the location of
the current limiter 205.
[0054] In the DC grids 100 shown in FIGS. 2 and 3, the current
limiters 205 have taken the place of the HVDC breaker 130 at one
end of the HVDC connection in which the current limiters 205 have
been connected. If a fault in the HVDC connection occurs in such
configuration, the current limiter 205 could advantageously operate
to break the current in order to interrupt the current flowing into
the fault. In one implementation, a protection system 135 is
provided which is arranged to instruct the current limiter 205 to
break the current in this situation. The control system for
controlling the limitation strength of the current limiter 205,
further discussed in relation to FIG. 5, should advantageously be
independent of such protection system 135, in order to improve
secure operation of the DC grid 100. In an alternative
configuration, a HVDC connection in which a current limiter 205 is
provided could be equipped with two HVDC breakers 130, in addition
to the current limiter 205. In this configuration, if fault the
HVDC connection were to occur, it would be sufficient for the
current limiter 205 to provide sufficient current limitation to
reduce the impact of the fault.
[0055] A high speed semi-conductor current limiter has been
described in EP0867998, and an example of such a current limiter
205 is shown in FIG. 4a. The current limiter 205 of FIG. 4a
comprises a set of n series-connected breaker sections 400, where
each breaker section comprises a parallel connection of a
non-linear, voltage-dependent resistor 410 and a semiconducting
switch 405 of turn-off type. The breaker sections 400 can be
controlled independently of each other. Here, a breaker section 400
having a semi-conducting switch 405 which is closed will be
referred to as a closed breaker section 400, and vice versa. The
different breaker sections 400 could be identical, although this is
not a requirement.
[0056] The current limiter 205 shown in FIG. 4a is bi-directional,
where the semi-conducting switch 405 is a series connection of two
anti-parallel, uni-directional switches of turn-off type, each
being connected in anti-parallel with a rectifying element (e.g. a
diode). Other ways of obtaining a bi-directional current limiter
205 may be contemplated, such as using a bi-directional 405, or
series-connecting two uni-directional current limiters in
anti-parallel to form the current limiter 205. As mentioned above,
a uni-directional current limiter 205 can advantageously be used in
some applications of the current limiter 205, in which case a
semi-conducting switch 405 could comprise one uni-directional
switch of turn-off type connected in anti-parallel with a
rectifying element. A semiconducting switch 405 of turn-off type
could for example be of IGBT type (insulated-gate bipolar
transistor), of IGCT type (integrated gate-commutated thyristor) or
of GTO (gate turn-off thyristor) type. All these types belong to
the group of power semiconductor switches with turn-on and turn-off
capability, and other switches belonging to this group could also
be used. (Typically, a semiconducting switch 405 is typically
formed be a series- and/or parallel connection of a number of
switching units.) The non-linear resistor 410 could for example be
an arrestor, and could be made from e.g. zinc oxide or silicon
carbide.
[0057] The fault current limitation functionality of the current
limiter 205 of FIG. 4a is obtained by blocking the switches 405 in
only a subset of the n sections (hereinafter, a breaker section 400
wherein the semi-conducting switch 405 is in a blocking state will
be referred to as an open breaker section 400). The non-linear
resistors 410 of the open section 400 set up a voltage which
counteracts the flow of current through the non-linear resistors
410. The higher number of sections 400 that are opened, the smaller
the current will be, with zero current as the extreme. By opening a
suitable subset of the n series connected breaker sections 400, the
counter voltage across the corresponding arrestors can be made
smaller than the voltage required to break the current, but large
enough to limit the current to a suitable level. However, as long
as the current is only limited (rather than broken), the non-linear
resistors 410 in the subset of open sections 400 will dissipate
energy. The number n of sections 400 and the Switching Impulse
Protection Level (SIPL) of each non-linear resistor 410 will
determine the maximum voltage for which a current may be broken. In
order to ensure that a fault current can be broken, the number n of
section 400 could advantageously reach or exceed the number
required for breaking a current at nominal voltage. However, if
only current limiting properties are desired, and no breaking
operation is expected from the current limiter 205, a smaller
number of sections 400 could be used. A current limiter 205 which
is capable of breaking the current can be referred to as a current
limiting breaker.
[0058] On occurrence of a fault, the current limiters 205 will
switch in non-linear resistors 410, to limit the current flowing
through the current limiter 205 so that it falls below a certain
level. As the current limiter 205 reduces the current by building
up voltage across the non-linear resistors 410 of the open
sections, the voltage at the side of the current limiter 205 will
be maintained at a voltage close to the normal voltage, instead of
dropping drastically.
[0059] FIG. 4b schematically illustrates an alternative embodiment
of a current limiter 205 of high speed, where the current limiter
205 of FIG. 4b comprises a transfer switch 415 which is connected
in parallel with the series connection of breaker sections 400. The
series connection of breaker sections 400 can in this configuration
be referred to as the main switch 417. The transfer switch 415
comprises a series connection of an auxiliary switch 425 and a
disconnector 420. During normal operation, the auxiliary switch 425
and the disconnector 420 are closed, so that the current through
the current limiter 205 flows through the transfer switch 415
rather than through the main switch 417. The sections 400 of the
main switch 417 can advantageously be open during normal
operation.
[0060] Upon activation of the current limiter 205, either to limit
or break the current, the auxiliary switch 425 will be opened so
that the current is commutated to the main switch 417. Before the
auxiliary switch 425 is opened, the main switch 417 should be
closed, if not closed during normal operation. When the auxiliary
switch 425 has been opened to commutate the current to the main
switch 417, the disconnector 420 will be opened, in order to
isolate the auxiliary switch from any high voltages which will
occur across the main switch 417. A suitable number of breaker
sections 400 will then be activated in that the semi-conducting
switch 405 of these breaker sections 400 will act to block the
current, thus forcing the current to flow via the non-linear
resistors 410. The main switch 417 of the current limiter 205 of
FIG. 4b should not be activated until the disconnector 420 has been
opened. Thus, a current limiter 205 which has a transfer switch 415
is generally slower than a current limiter 205 having a main switch
417 only. However, an indication that a fault has occurred will
often have to be analyzed before a decision to limit (or break) the
current will be taken. By using the time between receipt of such
fault indication and the decision making for a preparatory opening
of the transfer switch 415, the activation of the main switch 417
can often take place immediately upon a decision having been
made.
[0061] Advantageously, the opening of the transfer switch 415 could
be performed upon receipt of a fault indication, and the activation
of the main switch 417 upon receipt of a main switch activation
decision. If no such decision is received, e.g. within a certain
period of time, the transfer switch 415 could be closed.
[0062] The additional time required for opening of the transfer
switch 415 may be beneficial for protection algorithms based on
derivatives of voltages and/or currents, since the opening the
transfer switch 415 would provide a time window with quickly
changing currents and voltages, before current limitation occurs.
Protection algorithms based on time derivatives would then be given
time to identify the fault, and to send trip signals to the
appropriate DC breakers 130. When no transfer switch 415 is
provided, a time delay of appropriate duration could be introduced
for this purpose, if desired. For some protection algorithms, such
as differential protection algorithms, such time window of quickly
changing currents and voltages are of no additional benefit.
[0063] The disconnector 420 of a transfer switch 415 should
preferably be fast. Since there will be no current through the
disconnector 420 while opening, a fast mechanical disconnector is
somewhat easier to design than a fast mechanical breaker for
breaking a current. An example of a suitable design of the
disconnector 420 is disclosed in EP1377995.
[0064] By letting the current flow through the transfer switch 415
during normal operation, the power loss in the current limiter 205
can be considerably reduced as compared to a current limiter 205
having only the main switch 417. The auxiliary switch 425 can be
considerably smaller than the main switch 417, and thus
considerably less power consuming. However, in relation to the
present invention, the transfer switch 415 is optional.
[0065] An advantage of using current limiters 205 based on a series
connection of breaker sections 400 is that the current-limiting
strength can easily be adjusted. Furthermore, if a sufficient
number of breaker sections 400 are provided, such current limiter
205 can operate to break the current. However, this current limiter
type is given as an example only, and other types of current
limiting devices may be used, such as superconducting
conductors
[0066] FIG. 5 provides an illustration of an example of a current
limitation system 500 comprising a current limiter 205 and a
control system 502 for controlling the current limiter 205 which is
connected in a HVDC connection. The control system 502 comprises a
current measurement device 505 arranged to measure the current
through the current limiter 205 and to generate a signal I
indicative of the measured current. The control system 502 further
comprises a voltage measurement device 510 arranged on each side of
the current limiter 205, arranged to measure the voltage on each
side of the current limiter 205 and to generate a signal U.sub.1
and U.sub.2, respectively, indicative of the voltage on a first and
second side of the current limiter, respectively. The current
measurement device 505 could for example be an optical current
transducer (OCT) or a DC current feedback compensation transducer
(DCCT), or any other suitable current transducer or sensor. The
voltage measurement devices 210 could for example be direct voltage
divider, or any other suitable voltage measurement device. As will
be seen below, the voltage measurement devices and the inputs for
receiving the signals U.sub.1 and U.sub.2, respectively, could be
omitted in some embodiments of the control system 502.
[0067] The control system 502 further comprises a
limitation-determination system 515, arranged to receive signals I,
U.sub.1 & U.sub.2 and to generate a control signal 520 to be
delivered to the current limiter 205, the control signal 520 being
indicative of the number of breaker sections 400 to be
opened/closed. Thus, the input of the limitation-determination
system 515 of FIG. 5 is connected to the respective outputs of the
current measurement device 505 and the voltage control devices 510,
while the output of the limitation-determination system 515 is
connected to a control input of the current limiter 205.
[0068] The connections for transmitting the signals I, U.sub.1,
U.sub.2, 520 and 530 are typically wired connections in order to
obtain sufficient speed and reliability, although wireless
connections could also be contemplated.
[0069] The limitation-determination system 515 of FIG. 5 is shown
to include a limitation strength determination mechanism 535 and a
control signal generator 540. The limitation strength determination
mechanism 535 is arranged to determine whether the present
limitation strength of the current limiter 205 should be increased
or decreased. For the control of a current limiter 205 having a set
of series connected breaker sections 400 as shown in FIGS. 4a and
4b, the limitation strength determination mechanism 535 is arranged
to determine whether the number k of open breaker sections 400
should be increased or decreased. The limitation strength
determination mechanism 535 is furthermore arranged to deliver, to
the control signal generator 540, a signal 545, indicative of the
currently required limitation strength (or, alternatively, of a
variation in the required limitation strength). When the current
limiter 205 is a current limiter having n independently
controllable breaker sections 400, the signal 545 will be
indicative of the number k of breaker sections that should be open
(or, alternatively, of variations in the number k).
[0070] The control signal generator 540 is arranged to generate a
control signal 520 in response to a signal 545 indicative of a
change in the desired number k of open breaker sections 400. In
case of the current limiter 205 being breaker-section based (cf.
FIGS. 4a and 4b), the control signal generator 540 could
furthermore be arranged to select which of the breaker sections 400
should be open or closed. The operation of the control signal
generator 540 will be further discussed in relation to FIG. 9.
[0071] The current limiter 205 could advantageously be further
connected to a protection system 135, as shown in FIG. 5.
Protection system 135 is preferably independent of control system
502, and arranged to detect a fault 200 which would require that
the current limiter 205 was opened, i.e. that the current limiter
205 broke the current. Such fault 200 could for example be a line
fault along a HVDC connection in which the current limiter is
connected, or a fault in the DC switchyard 120 to which the current
limiter 205 is connected. Upon detection of such fault, protection
system 135 would send a tripping signal 530, in response to which
the current limiter 205 would break the current--in the current
limiters 205 of FIGS. 4a and 4b, this would involve sending a
tripping signal to the semiconducting switch 405 of each of the
breaker sections 400 (or at least, in case of redundant breaker
sections 400, to a sufficient number of breaker sections 400 to set
up a sufficient voltage to break the current).
[0072] In a configuration wherein a HVDC connection is equipped
with two HVDC breakers 130, in addition to the current limiter 205,
no protection system 135 has to be connected to the current limiter
205 the protection system 135. Instead, both HVDC breakers 130
could be connected to a protection system 135. In this
configuration, the number of sections 400 of a section-based
current-limiter 205 does not have to be sufficient to set up a
voltage capable of breaking the current, but the number of sections
could be designed for current-limitation scenarios only.
[0073] Although current limiters 205 of any suitable design could
be used in the present invention, it will hereinafter be assumed,
for illustrative purposes, that current limiters 205 based on
independently controllable breaker sections 400 are used.
[0074] The operation of different embodiments of the limitation
strength determination mechanism 535 will now be discussed. The
limitation strength of the current limiter 205, which is here
determined by the number k of open breaker sections 400, can for
example be controlled in dependence on measurement of the present
current I through the current limiter 205, i.e. in dependence on a
value conveyed to the limiting strength determination mechanism 535
by the signal I. An example of a determination process is
illustrated in FIG. 6a, wherein the number k of open breaker
sections 400 is controlled to regulate the current I through the
current limiter 205 to lie within a regulation range:
I.sub.min<|I|<I.sub.max. The regulation range represents a
desired current range in case of a fault--in case of a detected
fault, if the current lies above I.sub.max, at least one breaker
section 400 is opened (unless all are already open); and if a the
current lies below I.sub.min, at least one breaker section 400 is
closed (if any are open).
[0075] At step 600 of FIG. 6a, the process is initiated in that the
parameter k, indicating the number of breaker sections 400 that
should be open, is set to zero. Typically, this step is first
entered when the current limiter 205 is initiated for normal
operation. In step 605, it is then checked whether the present
current I exceeds the current threshold representing the maximum
current level of the regulation range, I.sub.max. If so, step 606
is entered, wherein it is checked whether the number of open
breaker sections 400, represented by the number k, exceeds zero. If
not, i.e. if no breaker sections 400 are open, the k is set to a
predetermined number k0, which can for example be chosen such that
the voltage across the k0 non-linear resistors 410 will
approximately correspond to the nominal voltage or the rated
voltage. Hence, if no breaker sections 410 are switched in upon
entry into step 605, a predetermined number k0 will be switched in
step 607. Step 615a is then entered, wherein a signal 545
indicative of k is generated and sent to the control signal
generator 540. Step 620a is then entered, wherein a delay is
introduced in order to let the current level adjust to the new
voltage situation. After the delay step 620a, step 605 is
re-entered.
[0076] If in step 606 it is found, on the other hand that k already
exceeds zero, then step 610 is entered, wherein the number k,
representing the desired number of open breaker sections 400, is
incremented by one. Step 615a is then entered.
[0077] If in step 605 it is instead found that the present current
level I lies below I.sub.max, then step 625 is entered, where it is
checked whether I is below a current threshold representing the
minimum current of the regulation range, I.sub.min. If not, no
action is required and step 605 is re-entered. However, if the
present current level I is found to lie below I.sub.min, then step
630 is entered, wherein it is checked whether k has a value larger
than 0. If not, the limitation strength of the current limiter 205
cannot be reduced further, and step 605 is therefore re-entered.
However, if k>0, indicating that at least one breaker section
400 is open, then step 640 is entered, wherein the value of k is
reduced by one. Step 615b is then entered, wherein a signal 545
indicative of k is generated and sent to the control signal
generator 540, prior to entering the delay step 620b. After step
620b, step 605 is re-entered.
[0078] Once a fault in a has been disconnected by HVDC breakers 130
on either side of the fault, the DC voltages in on the faulty side
of the current limiter 205 will start to increase and the
fault-current contribution from surrounding parts of the DC grid
100 will tend to go below the lower threshold I.sub.min, at which
time the open breaker sections 400 will start to close to maintain
the current within the two thresholds. Once all sections are
closed, normal operation is resumed.
[0079] The regulation range [I.sub.min, I.sub.max] could be set to
lie entirely above the rated current of the transmission,
I.sub.rated; the regulation range could be set to lie entirely
below the rated current, I.sub.rated; or the regulation range could
be set so that I.sub.rated falls within the regulation range. The
rated current of the transmission, I.sub.rated, is here typically
the rated current of the HVDC connection in which the current
limiter 205 is connected, depending on the dimensioning of the
components of the DC grid 100.
[0080] In the current regulation process illustrated in FIG. 6a,
the present level of the current I is the determining factor for
whether or not the limitation strength of the current limiter 205
should be altered. If I lies below the minimum current of the
regulation range and there are no open breaker sections 400, then
no further action will be taken to amend the current. In other
words, if the current level is at or below the rated current, the
current limiter 205 will not act to amend the current level when
the regulation range lies above the rated current. Hence, if a
fault has occurred and later been cleared, the process in FIG. 6a
would operate to return the present current level to the current
level during normal operation. Thus, the current regulation process
of FIG. 6a is suitable if the rated current I.sub.rated lies below
the regulation range. As can be seen in FIG. 6a, no information on
the voltage on either side of the current limiter 205 is required,
and thus, when the regulation range lies entirely above the rated
current, the voltage measurement devices 610 could be omitted from
the control system 502. However, if desired, voltage measurements
as well as current measurements could be used to detect a fault
situation.
[0081] However, when the rated current lies above the regulation
range, the situation is different, and the fact that the current
has exceeded the maximum current of the regulation range is not an
appropriate indication that the current limiter 205 should be
activated. The method illustrated in FIG. 6a would in this
situation regulate the current to lie below the rated current
(within the regulation range), upon occurrence of a fault, as well
as during normal operation before any fault has occurred &
after a fault has been cleared. In order to avoid such undesired
suppression of the current during normal operation, an additional
condition could be included in the method of FIG. 6a. Such
additional condition could for example be based on the voltage
level on one or both sides of the current limiter 205, depending on
whether the current limiter is uni-directional or
bidirectional.
[0082] An embodiment of the regulation process discussed in
relation to FIG. 6a, to which a check of such additional condition
has been added, is shown in FIG. 6b. The embodiment of FIG. 6b
comprises step 603, which is entered after step 600, prior to
entering step 605. In step 603, it is checked whether both the
voltage U.sub.1 on a first side of the current limiter and the
voltage U.sub.2 on the second side of the current limiter 205
exceed a lower voltage level U.sub.low. If so, it is concluded that
no fault has occurred which requires current limitation, and step
600 will be re-entered. However, if it is found in step 603 that
the voltage level on either side of the current limiter 205 (or
both sides) has fallen below the lower voltage level U.sub.low,
this is an indication that a fault has occurred and that the
current limiter 205 should be activated. Step 606 is then entered.
U.sub.low could for example lie within the range
0.1U.sub.rated<U.sub.low<0.8U.sub.rated. In step 606, it is
checked whether k exceeds zero. If not, i.e. if no breaker sections
400 are open, then k is set to a predetermined number k0, which can
for example be chosen such that the voltage across the k0
non-linear resistors 410 will approximately correspond to the rated
voltage. Step 615a is then entered, wherein a signal 545 indicative
of k is generated and sent to the control signal generator 540.
Step 620a is then entered, wherein a delay is introduced in order
to let the current level adjust to the new voltage situation. After
the delay step 620a, step 603 is re-entered.
[0083] If it is found in step 606, on the other hand that k already
exceeds zero, then step 605 is entered, wherein the number k,
representing the desired number of open breaker sections 400, is
incremented by one. Step 615a is then entered.
[0084] Once step 605 has been entered, the procedure is similar to
the procedure of FIG. 6a. However, rather than re-entering step 605
after the delay in step 620a/620b, or after a negative conclusion
in step 625 or 630, step 603 is re-entered to ensure that the fault
is still present. Thus, in the fault-current-strength determination
method of FIG. 6b, the voltage level at the current limiter 205 is
used as an indicator of a fault situation, while in the method of
FIG. 6a, the current level through the current limiter 205 is used
as such indicator. As can be seen, in an embodiment wherein the
desired fault-current range lies below the rated current,
measurements of the voltages U.sub.1 and U.sub.2 on the respective
sides of the current limiter 205 are useful.
[0085] If it is concluded in step 603 that the voltage on both
sides exceeds U.sub.low, this indicating that the fault has been
cleared, the limitation-strength indicator k is set to zero in step
600. Thus, the current limiter 205 is deactivated. In an
alternative implementation, the limitation-strength indicator k
will be gradually be reduced to zero after a finding in step 603
that the voltage exceeds U.sub.low.
[0086] The regulation method of FIG. 6b is suitable also when the
regulation range includes the rated current of the transmission,
I.sub.rated. By introducing the condition of step 603, it is
ensured that the current limiter 205 will be inactive during normal
operation. If desired, the regulation method of FIG. 6b could be
used also when the regulation range lies below the rated current
I.sub.rated. As will be discussed in relation to FIG. 9, the
regulation range could include the extreme case where the current
is limited to zero.
[0087] In the regulation methods illustrated in FIGS. 6a and 6b,
the magnitude of the current and/or voltage is used as a basis for
determining whether a fault has occurred, and thus whether or not
to limit the current. The direction (sign) of the current, on the
other hand, can advantageously be used in order to determine in
which direction a closed breaker section 400 should be blocked, in
case a bi-directional current limiter 205 is used.
[0088] To regulate the fault current through a current limiter 205
so that it falls within a regulation range above the rated current
level has the advantage of providing a higher charge of the cables
and/or overhead-lines forming the HVDC lines 115 in the DC grid 100
once the fault has been cleared. This is particularly advantageous
when the current limiter 205 is connected as a zone-dividing
current limiter 205 to define a zone border.
[0089] To regulate the fault current through a current limiter 205
so that it falls within a regulation range below the rated current
level, on the other hand, results in a lower power loss in the
current limiter(s) 205, thus allowing for a less heat-resistance
design of the current limiter(s) 205 (e.g. a simpler design of the
non-linear resistors 410 of the current limiters 205 shown in FIGS.
4a and 4b) and/or for a longer period of time during which a
current limiter 205 can carry a current (thus facilitating for the
use of slower HVDC breakers 130). A lower fault-current level also
yields a lower requirement on current-breaking capacity of the HVDC
breakers 130.
[0090] When the current limiter 205 is located in a connection 110
connecting a VSC converter 105 to the DC grid 100, the maximum
current of the regulation range, I.sub.max, could advantageously
lie below the converter blocking-level, so that the switches of the
VSC converter can stay in operation also during a fault-on period.
Furthermore, it would be advantageous to set the maximum current of
the regulation range to lie below the rated current when the
current limiter 205 is located in a connection 110 of a VSC
converter 105. By regulating the current in case of a fault so that
it falls below the rated current, it can be ensured that the active
power delivered to the DC grid during the fault falls below the
rated power of the VSC converter, thus allowing VSC converter to
continue to control the reactive power exchange with the AC power
system also during the fault-on period, and thus to provide support
of the AC system voltage. This will be beneficial for AC system
stability. The amount of reactive power that a HVDC converter 105
will be capable of delivering to the connected AC power system will
depend on the difference between the fault current through the
connection 110 and the rated current of the HVDC converter 105--the
larger this difference, the higher the amount of reactive power
that can be delivered to the connected AC power system, thus
facilitating for efficient control of the AC voltage in a manner so
that the disturbances on the connected AC power system resulting
from the fault in the DC grid will be minimized.
[0091] Depending on which of the above advantages is/are most
desired in a particular application of a current limiter 205, the
regulation range could be selected to lie above or below the rated
line current, or to include the rated current, I.sub.rated.
[0092] The processes illustrated by FIGS. 6a and 6b are examples
only and could be varied in different ways. For example, the
increment of k made in step 610 could differ from one, and could
for example depend on the difference between I.sub.max and the
present value of I--if the difference is large, then k could be
incremented by a higher number of steps than if the difference is
small. Similarly, the decrease of k in step 640 could be larger
than one, and could e.g. be made dependent on the difference
between I.sub.min and the present value I. Moreover, the signal 545
could be indicative of the desired change in k, rather than of k
itself. Furthermore, in another type of current limiter 205, step
610 (640) could represent another means of increasing (decreasing)
the limitation strength of the current limiter 205. The parameter k
in FIG. 6a is used to represent the number of open breaker sections
400. However, in the general case, the parameter k represents a
measure of the present limitation strength of the current limiter
205, and can be referred to as a limitation-strength indicator. The
signal 545 could be referred to as a limitation-strength signal
545.
[0093] Step 606 and 607 of FIGS. 6a and 6b serve to give the
current limitation a kick start, by opening a pre-determined number
of breaker sections. In another implementation, k can for example
be given a value in step 607 which depends on the time derivative
of the current and/or voltage. In yet another implementation, steps
606 and 607 could be omitted, and the current-limiting strength
could be determined in steps 610 and 640 only.
[0094] The delay steps 620a and 620b could be implemented as
identical steps. However, depending on the inductance of the DC
grid 100, it could be beneficial to use different durations of the
delay in the cases of incrementing (620a) and decrementing (620b)
the limitation strength indicator k. It might for example be
beneficial to have a shorter delay when the current lies above
I.sub.max, so as to ensure that a fault current does not rise at an
undesired rate, whereas when the current is decreased, it might be
beneficial to use a longer delay period, so that the current can
stabilize and unnecessary switching in-and-out of the current
limitation strength. Hence, in one embodiment, the duration of the
delay is shorter in step 620a than in step 620b. As a non-limiting
example, the duration of the delay in steps 620a and 620b could lie
within the range of 50 .mu.s-10 ms. However, other durations of the
delay could be used.
[0095] In one implementation of the invention, a reactor 700 is
connected in series with the current limiter 205, as shown in FIG.
7. By connecting a reactor 700 in series with the current limiter
205, the time derivative of the current will be reduced. This could
for example be beneficial in a scenario where a fault has occurred
in a location such that the inductance of the fault-current path is
low, and where none of the available values of k will keep the
fault current within the regulation range. A reactor 700 in series
with the current limiter 205 could in this scenario prevent a high
frequency of switching between different values of k. Since the
switching of semi-conducting switches 405 typically generates heat,
it might for cooling purposes be desirable to keep the switching
frequency low. The inductance of a reactor 700 could for example
lie within the range from around ten to some hundred mH.
[0096] The different embodiments of the process performed by the
limitation strength determination mechanism 535 shown in FIGS. 6a
and 6b only relate to the operation of a main switch 417 of a
current limiter 205. However, as discussed in relation to FIGS. 4a
and 4b, it is often useful to use current limiters 205 further
comprising a transfer switch 415. The process described by the
embodiments in FIGS. 6a and 6b applies also to a current limiter
205 having a transfer switch 415, once the transfer switch 415 has
been opened and the current has been commutated to the main switch
417.
[0097] An embodiment of a process for opening the transfer switch
415 is illustrated in FIG. 8a. The process of FIG. 8a could
advantageously be used together with the process shown in FIG. 6a
for determining the limitation strength when the regulation range
lies above the rated current of the transmission.
[0098] The process of FIG. 8a is based on the idea that the
transfer switch 415 should be opened (corresponding to "arming" the
current limiter 205) when a first indication of a fault is
detected, where the first indication is received at an earlier
stage than the fault detection of step 605. This fault indication
is therefore generally less certain, but since opening the transfer
switch 415 does not affect the operation of the DC grid 100 other
in that the power consumption increases during a short period of
time, an incorrect opening of the transfer switch is
acceptable.
[0099] At step 600 of FIG. 8a, the parameter k is first set to
zero, as discussed above in relation to FIG. 6a. Step 800 is then
entered, wherein it is checked whether the present current level is
below the arming current level, I.sub.arm. If so, step 800 is
re-entered. However, if the magnitude of the present current has
risen above I.sub.arm, step 805 is entered, wherein the transfer
switch 415 is opened. In step 810a, it is checked whether the
present current is below I.sub.arm and the limitation-strength
indicator k takes the value zero. If so, the transfer switch 415 is
closed, and step 800 is re-entered. If not, the main switch
activation decision step is entered (cf. step 605 of FIG. 6a),
wherein the process of determining the appropriate limiting
strength of the current limiter 205 is commenced.
[0100] Step 810a can be seen as superfluous when entered directly
after step 805, and can then be omitted. However, step 810 can
advantageously be entered also after having found, in step 630,
that the limitation-strength indicator k is zero while the current
lies below the minimum current in the regulation range. In this
situation, the current-limiting functionality of the current
limiter 205 is no longer active, and a check as to whether the
transfer switch 415 should be closed could advantageously be made.
In this situation, if the current still lies above I.sub.arm, it
would be advantageous to keep the transfer switch 415 in an open
state, in order to quickly be able to limit the current again, if
needed. However, if I.sub.arm lies above the minimum current of the
regulation range, or if such pre-cautionary maintaining of the
transfer switch 415 is not desired, step 815 could be entered
directly after having determined in step 830 that k has taken the
value zero.
[0101] FIG. 8b illustrates an example of a process for opening of
the transfer switch 415 which could for example be used together
with the process shown in FIG. 6b for determining the limitation
strength when the regulation range lies below or partly below the
rated current of the transmission. The process of FIG. 8b is
similar to that of FIG. 8a. After the limitation-strength indicator
has been set to zero in step 600, step 800 is entered, wherein it
is checked whether the current lies below the arming current,
I.sub.arm. If so, step 800 is re-entered. If the current level is
larger than I.sub.arm, the transfer switch is opened in step 805.
Step 810b is then entered, wherein it is checked whether the
current level lies below the arming current. If so, step 815 is
entered, wherein the transfer switch is closed. Step 800 is then
re-entered. However, if it is found in step 810b that the current
lies above I.sub.arm, the main switch activation decision step is
entered (cf. step 603 of FIG. 6b), wherein the process of
determining the appropriate limiting strength of the current
limiter 205 is commenced.
[0102] Similar to FIG. 8a, step 810b could be omitted if entered
directly after the transfer switch has been opened in step 805.
However, step 810b could advantageously be entered also when it has
been determined in step 603 that the voltage at both sides of the
current limiter 205 lies above the lower voltage level U.sub.low.
In this situation, the current-limiting functionality of the
current limiter 205 should be de-activated, and a check as to
whether the transfer switch 415 should be closed could
advantageously be made. A step 820 could be introduced, which is
entered from the y-branch of step 603, prior to entering step 810b.
In step 820, the current-limiting strength indicator k is set to
zero. Hence, by entering step 820 instead of step 600 after a
positive decision in step 603, steps 800 and 805 does not have to
be performed in a situation where the transfer switch 415 will
definitely be open. In FIG. 8b, the step 810 differs from step 810a
of FIG. 8a in that no check as to whether k is zero is performed,
since k has been set to zero in step 820. However, a k-check could
be made also in step 810b, if desired.
[0103] The procedures of FIGS. 8a and 8b are examples only, and
could be altered in different ways. For example, in one
implementation of FIG. 8a, the check of step 810a is omitted, and
step 815 is entered directly after a negative decision in step 630.
Similarly, the check of step 810b could be omitted in FIG. 8b, and
step 815 could be entered directly after a positive decision in
step 603--in this implementation, step 820 could either be
included, or omitted.
[0104] In FIGS. 8a and 8b, the first indication of a fault is
represented by the present current level, I, raising above an
arming current level, I.sub.arm, I.sub.arm could e.g. in the range
I.sub.rated<I.sub.arm<2I.sub.rated. An alternative
representation of the first indication of a fault could be that the
voltage at either side of the current limiter 205 falls below an
arming voltage level, U.sub.arm. U.sub.arm could for example lie in
the range 0.5U.sub.rated<U.sub.arm<0.8U.sub.rated.
[0105] If desired, a different threshold could be used for closing
the transfer switch 415 than for opening the transfer switch 415,
so that the threshold of step 810a/810b is higher than that of step
800 when the current is used as a first indication of a fault, or
lower than that of step 805 when the voltage is used as a first
indication of a fault.
[0106] An embodiment of a process performed by control signal
generator 540 is schematically illustrated in the flowchart of FIG.
9. At step 900, a limitation-strength signal 545 is received from
the limitation strength determination mechanism 535. At step 905,
the required action is determined in dependence on the
limitation-strength signal (other information may also be used in
the determining process). When the current limiter 205 is based on
a series-connection of breaker section 400, step 400 involves
determining the number of breaker sections 400 that should be open.
The determination typically also includes determining which breaker
section(s) 400 that should be opened or closed. In step 910, a
control signal 520 is then generated at the output of the
limitation-determination system 515, to which the current limiter
205 is connected. When the current limiter 205 comprises
semiconducting switches 405, such control signal 520 could for
example comprise a combination of firing and/or blocking signals
(depending on whether breaker sections 400 should be switched in or
out) in a conventional manner, so that each semiconducting switch
405 that should change its state will receive a firing or blocking
signal.
[0107] The determination of step 905 could for example be based on
a predetermined scheme for opening/closing the breaker sections
400. Such pre-determined scheme could for example operate to open
(close), when the current-limiting strength is to be increased
(decreased), the semi-conductor switch 405 of the breaker section
400 that has been closed (open) the longest of the closed (open)
breaker sections 400. In such embodiment, the control signal
generator 540 could for example include a memory for storing
information on at what moments the different breaker sections 400
were last switched in or out. Other predetermined schemes could
alternatively be used.
[0108] Alternatively, in a breaker section based current limiter,
the control signal generator 540 could in step 905 determine which
breaker sections 400 to open or close based on an estimation of the
temperature of, or the amount of energy absorbed in, the different
non-linear resistors 410, so that the open breaker sections 400
will be selected from the breaker sections 400 having the lowest
temperature, or, correspondingly, the highest energy-absorbing
capacity. The highest temperate that is safe, or the maximum energy
that can safely be absorbed by a non-linear resistor 410 during a
fault-on period is typically known (the effects of cooling can
typically be neglected during the fault-on period). The present
energy-absorbing capacity of different non-linear resistors 410
could then for example be estimated by calculations of the absorbed
energy in the non-linear resistors 410, or by means of measurements
performed by a temperature sensor which would be arranged to
deliver a temperature signal to the control signal generator 540.
For example, the following expression could be used for the
estimation of the energy absorbed in a non-linear resistor 410:
E.sub.410,i(t)=.intg..sub.t.sub.start.sup.tI(t)U.sub.410,i(I)a.sub.i(t)d-
t (1a),
where E.sub.410,i(t) is the energy absorbed by the i.sup.th
non-linear resistor 410 at time t since the occurrence of the fault
at time t.sub.start; I(t) is the current through the
current-limiting breaker 205, which is measured by the current
measuring device 505 and known to the current limiting system 515;
U.sub.410,i(I) is the known U-I-characteristic of a non-linear
resistor 410; and a.sub.i(t) is a function which takes the value 0
when the semi-conducting switch 405 of the i.sup.th breaker section
is closed, and the value 1 when the semi-conducting switch 405 of
the i.sup.th breaker section 400 is open. Expression (1a) could be
refined, if desired, to for example include effects of cooling.
However, during a fault-on period, effects of cooling can generally
be neglected, since time constants for cooling are typically much
longer than the fault on period. Furthermore, an estimation of the
absorbed energy E.sub.410,i(t) based on an assumption that the
voltage U.sub.410,i across a non-linear resistor is constant will
in most applications give an estimation of sufficient accuracy:
E.sub.410,i(t)=.intg..sub.t.sub.start.sup.tI(t)U.sub.410,ia.sub.i(t)dt
(1b).
[0109] When expression (1) is used for determining which breaker
sections 400 to open or close, the breaker section 400 to be opened
when k is increased could for example be the breaker section 400
having the lowest E.sub.410,i(t) of the currently closed breaker
sections 400, and breaker section to close when k is decreased
could for example be the breaker section 400 having the highest
E.sub.410,i(t) of the currently open breaker sections 400.
[0110] Following fault clearing, the estimate of the energy
absorbed in the non-linear resistors 410 should advantageously be
adjusted to reflecting cooling, so that an accurate estimate of the
absorbed energy is available should another fault occur. In one
implementation, this is solved by only allowing re-closing the
current limiter 205 after a cooling time period has elapsed since
the current limiter 205 was activated, at which time the
E.sub.310,i(t) is re-set to the initial value.
[0111] If an indication exists that the current limiter 205 may be
damaged unless the current is actually broken rather than limited,
the determination of step 905 could advantageously result in a
decision to open enough (typically all) breaker sections 400, in
order to break the current. Such damage-indication could for
example be based on an estimation of absorbed energy in the
non-linear resistors 410; on temperature measurements of the
non-linear resistors 410; or on the time during which the
non-linear resistors 410 have been switched in during the fault-on
period. The procedure to asses when to trip the current limiter 400
due to excessive absorbed energy could for example be based on the
breaker section 400 having the highest absorbed energy. Assuming
that breaking the current completely would require all breaker
sections 400, it should be ensured that the breaker section 400
with the highest absorbed energy can be switched-in a final time.
Thus, in one embodiment, the current limiter 205 is tripped when
the absorbed energy of a non-linear resistor 410 reaches an energy
threshold, and no non-linear resistor 410 of another breaker
section 400 could be switched in to take its place if switched out.
The energy threshold could be set with a margin to the energy level
at which the non-linear resistor 410 will be damaged. In opening
all breaker sections 410, the fault-current contribution to the
fault from the healthy part of the DC grid connected on the other
side of the current limiter 205 will be extinguished.
[0112] If a current limiter 205 has been tripped in order to
protect the non-linear resistors from thermal damage, the control
system 502 could, in one embodiment, continue to monitor the
voltages U.sub.1 and U.sub.2 at the current limiter 205 (or, only
U.sub.1 or U.sub.2, in case of uni-directional limiter). In this
embodiment, a process such as the one shown in of FIG. 6b could be
entered upon such self-protective tripping of the current limiter
205, with a minimum current of the regulation range, I.sub.min, set
to zero. This would particularly be useful if the faulty part of
the DC grid is connected to another current source, for example an
HVDC converter 105, the current supply of which has not been cut
off. When the fault is cleared, the voltage which was depressed
during the fault-on period will then start to rise, providing an
indication to the current limiter 205 that the fault has been
cleared.
[0113] The estimation of absorbed energy used to obtain a
damage-indication could e.g. be made in accordance with expression
(1a) or (1b). However, in order to ensure that the current can
actually be broken in case a fault occurs when another fault has
recently been cleared, cooling of the non-linear resistors 410
should advantageously be taken into account. This could for example
be solved by only allow re-closing the current limiter 205 after a
cooling time period has elapsed since the current limiter 205 was
activated, such that a re-opening of the current limiter 205 can be
performed without damaging the non-linear resistors. The time
constant for cooling could, in one implementation, be in the order
of an hour. Alternatively, the expression used for estimation of
absorbed energy could be refined to include the effects of cooling.
A self-protective control system which is arranged to generate a
damage indication and, if required, a self-protective tripping
signal, could for example be implemented as part of control system
502, or as part of an independent protection system 135. In an
embodiment wherein the current limiter 205 is not capable of
breaking the current, such damage indication could be used to
trigger the tripping of an HVDC breaker 130 protecting the current
limiter 205.
[0114] In order to ensure that the current limiter 205 would not be
damaged in the unusual event of the current limiter not being
capable of tripping when the self-protection system gives a
tripping instruction, a redundant current limiter 205 could be
provided, or, when the current limiter 205 is based on a
series-connection of breaker sections 400, redundant breaker
sections 400 could be provided in the current limiter 205.
Alternatively, the HVDC connection could be short-circuited so that
the current limiter 205 is bypassed, thus leaving the clearing of
the fault to HVDC breakers 130 or current limiters 205 elsewhere in
power system.
[0115] In FIG. 10, an alternative way of schematically illustrating
the limitation-determination system 515 of FIG. 5 is shown, wherein
the limitation-determination system 515 is implemented by using a
combination of hardware and software. FIG. 10 shows the
limitation-determination system 515 comprising processing means
1000 connected to a computer program product 1005 in the form of a
memory, as well as to interfaces 1010 and 1015. Interface 1010 is
arranged to receive input signals comprising information relevant
to the limitation strength determination. Such signals include
signal I indicative of the present level of the current, and could
e.g. also include signals U1 or U2 (or both, as appropriate) and
signals indicative of the temperatures of the non-linear resistors
410, as discussed above. Interface 1015 is arranged to deliver the
control signal 520.
[0116] The memory 1005 stores computer readable code means in the
form of a computer program 1020, which, when executed by the
processing means 1000, causes the limitation-determination system
515 to perform a current-limiting control method. Different
embodiments of such method are illustrated in FIGS. 6a-b, FIG. 8
and FIG. 9. In other words, the limitation-determination system 515
would in this embodiment be implemented by means of one or more
general purpose processors or one or more processors especially
developed for the limitation-determination system 515, in
combination with software 1020 for performing current limiting
control. In FIG. 10, the software 1020 is shown to be stored on one
physical memory 1005, however, software 1020 could be divided onto
more than one physical memory 1005. A memory 1005 could be any type
of non-volatile computer readable means, such as a hard drive, a
flash memory, an EEPROM (electrically erasable programmable
read-only memory) a DVD disc, a CD disc, a USB memory, etc.
[0117] FIGS. 11a-11d illustrate the sequence of events according to
some embodiments described above, in a situation where a line fault
occurs in a first zone 300 at a time t.sub.f, and where the fault
is cleared at a time t.sub.c. The current I through a current
limiter 205 is plotted against time, as well as the voltage
(U.sub.1) at side of the current limiter 205 which is connected
towards the fault, and the voltage (U.sub.2) at the other side of
the current limiter 205, connected to the healthy part of the DC
grid 100. FIGS. 11a and 11b represent embodiments wherein the
current limiter 205 does not have a transfer switch 415, while
FIGS. 11c and 11d represent embodiments wherein a transfer switch
415 is present. Furthermore, FIGS. 11a and 11c represent
embodiments wherein the rated current lies below the regulation
range, while FIGS. 11b and 11d represent embodiments wherein the
rated current lies above the regulation range. Worth noting is that
the voltage in the healthy part of the DC grid, U.sub.2, is
essentially undisturbed, making it possible to continue organized
power transfer in this part also during the fault-on period.
[0118] The application of current limiters 205 as discussed above
can be applied to both mono-polar and bipolar HVDC connections. If
the HVDC connection consists of two pole lines, with positive and
negative pole voltage, the HVDC connection will be equipped with
two current-limiters 205, while for a single pole HVDC connection,
positive or negative pole voltage, with or without a metallic
return, a single current limiter 205 will typically be used on the
single pole connection. Other configurations can alternatively be
used.
[0119] The above description has been made in terms a high voltage
DC grid. However, the invention is equally applicable to DC grids
comprising AC/DC converters of any voltage level, including Medium
Voltage Direct Current (MVDC) grids comprising MVDC converters,
MVDC connections and MVDC breakers.
[0120] Although various aspects of the invention are set out in the
accompanying independent claims, other aspects of the invention
include the combination of any features presented in the above
description and/or in the accompanying claims, and not solely the
combinations explicitly set out in the accompanying claims.
[0121] One skilled in the art will appreciate that the technology
presented herein is not limited to the embodiments disclosed in the
accompanying drawings and the foregoing detailed description, which
are presented for purposes of illustration only, but it can be
implemented in a number of different ways, and it is defined by the
following claims.
* * * * *